Among refractory products, a distinction is made between fused products, which are well known for the construction of glass furnaces, and sintered products.
Unlike sintered products, fused products usually comprise an intergranular vitreous phase binding the crystallised grains. The problems raised by sintered products and by fused products, and the technical solutions adopted to solve them, are therefore generally different. A composition developed to manufacture a sintered product is therefore not basically usable as such to manufacture a fused product, and vice versa.
Fused products, often called “electrofused” products, are obtained by melting a mixture of suitable batch materials in an electric arc furnace or by any other technique suitable for these products. The bath of molten material is then cast in a mould and the product obtained undergoes a controlled cooling cycle.
Among fused products, AZS electrofused products, that is mainly comprising alumina (Al2O3), zirconia (ZrO2) and silica (SiO2), have been known for decades. U.S. Pat. No. 2,438,552 describes one of the first improvements made to this type of product. The authors recommend adding Na2O (1-2.2%) and MgO/CaO (0.2-0.8%) to solve problems of feasibility concerning products comprising less than 70% Al2O3, from 14 to 40% ZrO2 and from 9 to 12% SiO2.
EP 0 939 065 B1 proposes to reduce the degree of exudation of AZS products containing 20 to 59% zirconia by adding B2O3, P2O5 and at least one of the oxides of the group SnO2, ZnO, CuO and MnO2. Adding the said oxides, which are not present as impurities in conventional starting materials, leads to production overcosts and possibly to problems with staining.
FR 2 875 497 proposes to reduce the degree of exudation of AZS products by selecting a chemical analysis such that ZrO2: 15.5 to 22%, SiO2: 10.5 to 15% and Na2O+K2O+Li2O: 1.0 to 2.5%, in particular if the Al2O3/ZrO2 weight ratio is equal to or higher than 2.9, and if the SiO2/(Na2O+K2O+Li2O) weight ratio is higher than 7 and/or lower than 9.
French application FR 08 58914 or application WO 2010/073195 proposes to improve the corrosion resistance of AZS products by adding Y2O3 for a chemical analysis such that ZrO2: 30 to 46%, SiO2: 10 to 16% and Na2O+K2O: 0.5 to 5%.
In all the examples according to the invention of these applications (Examples 4 to 10), SiO2/(Na2O+K2O+B2O3)<5×Y2O3. In contrast, none of the examples which meet the criteria SiO2/(Na2O+K2O+B2O3)≧5×Y2O3, except for Example 11, conforms to the invention of the said applications.
AZS products which are sold today by Saint-Gobain SEFPRO, such as ER-1681, ER-1685, and ER-1711, contain 45 to 50% Al2O3, 32 to 41% ZrO2, 12 to 16% SiO2 and about 1% Na2O. ER-2001 SLX contains about 68% Al2O3, 17% ZrO2, 13% SiO2 and 1.7% Na2O.
These products are suitable for manufacturing glass furnaces. More particularly, current commercially available AZS products are mainly used for the zones in contact with the glass melt and for the superstructure of glass furnaces.
These products are efficient but a permanent need exists to improve the conditions for controlling the glass furnaces and the quality of the glass.
In particular, for blocks of AZS fused product constituting the vessels of glass furnaces, it is important to limit the problems of exudation at the operating temperatures.
A need therefore exists for an AZS fused product having good resistance to corrosion by the glass melt and not having any problem of exudation at the operating temperatures of the vessels of glass furnaces.
It is the object of the present invention to satisfy this need.